COMPARISON BETWEEN TOMOGRAPHIC STRUCTURES AND MODELS OF CONVECTION INTHE UPPER-MANTLE

Gruneisen's and third-order finite-strain theories are used to compute the density and seismic-wave velocities of minerals. Assuming the min eralogical model of Ito & Takahashi (1987), seismic velocities of the upper mantle are calculated using the Hashin-Shtrikman averaging proce dure. 1-D profiles are first obtained along adiabats, and compared to the IASP91 model. Different adiabats are considered in order to take i nto account the thermal effect of phase transitions. The best results are found with adiabats initiated at 1473, 1573 and 1613 K for alpha-o livine, beta- and gamma-spinel, respectively. The incorporation of the rmal effects resulting from phase transitions gives velocity jumps at discontinuities close to those of IASP91. Next, a model of convection constructed by Dupeyrat, Sotin & Parmentier (1995), incorporating plat e tectonics, is used to compute 1-D profiles and 2-D fields of seismic anomalies in the upper mantle. Averaged profiles show seismic-velocit y gradients very close to those of IASP91, but individual values are m uch too high, suggesting that the mean temperature profile of the conv ection model is too cold by 400 K. When lowpass filtered to the resolu tion scale of presently available tomographic models, both the amplitu de and shape of the computed seismic anomalies are consistent with the results of tomographic studies. The amplitude of the anomalies ranges between -2.7 and 3.8% for P-wave slownesses, and from -3.3 to 4.5% fo r S-wave slownesses. These anomalies correspond to lateral temperature variations of -465 to 520 K. These calculations are used (1) as an ai d to the interpretation of global tomographic models, for instance by computing spectra of lateral heterogeneities, and (2) to test the adeq uacy of the basic assumptions used in the computation of numerical mod els of mantle convection, and to build a theoretical temperature profi le that would give the best fit to IASP91. In the uppermost mantle thi s theoretical model has a shape close to both the convection model and the 1473 K adiabat, but in the transition zone the profile is highly subadiabatic. The spectra obtained for the synthetic seismic anomalies resemble that of tomographic studies, with most of the energy contain ed within gravest angular orders l, and a fast decrease of energy with increasing l. The spatial filtering has clearly different effects on heterogeneities, depending on their respective wavelengths. It is sugg ested that the change of decreasing rate observed in tomographic model s at l = 7 is closely related to the filter wavelength and may corresp ond at a lesser extent to a characteristic wavelength of mantle hetero geneities.